Oxidative DNA damage presents a serious challenge to genomic integrity and can accelerate carcinogenesis and aging. Reactive oxygen species-dependent DNA damage are repaired by base excision repair (BER), initiated with removal of base lesions by DNA glycosylases. The overall goal of this project is to study the role of selected DNA glycosylases and mismatch repair enzymes in response to cellular oxidative stress. We focus on the role of human MutY homolog (hMYH) glycosylase and its interactions with cell cycle and aging regulators. hMYH reduces stress-induced mutagenesis by removing misincorporated adenines paired with 8- oxoG (the most abundant form of DNA damage), therefore reduces G:C to T:A mutations. hMYH deficiency predispose individuals to colon cancer. hMYH interacts with the DNA replication machinery, other repair enzymes, the 9-1-1 cell cycle checkpoint complex (Rad9/Rad1/Hus1), and the aging regulator SIRT6. In this context, the 9-1-1 proteins interact with and increase the activities DNA glycosylases and hMSH2/hMSH6 mismatch recognition complex. We hypothesize that MYH and other DNA repair enzymes serve as molecular adaptors to recruit checkpoint proteins to DNA lesion sites and coordinate DNA repair and increase repair efficiency and fidelity. To examine this hypothesis, we propose three specific aims: (1) The dynamic interaction of MYH with MSH2/MSH6 both in vitro and in vivo will be delineated. We will test whether these interactions are altered following oxidative stress and during the progression of the cell cycle. (2) The physical and functional Interactions of MYH (and Neil1, hOGG1 and hMSH2/hMSH6) with the 9-1-1 complex will be elucidated. We will investigate how different proteins compete for the 9-1-1 complex and why different glycosylases select different subunits of the 9-1-1 complex. The biological significance of Hus1-MYH interaction will be investigated by interruption of the interaction by mutagenesis and by using a Hus1 binding competitor peptide. We will test a model that DNA glycosylases act as adaptors to recruit the 9-1-1 complex to the lesion sites. (3) The novel role of an aging regulator SIRT6 in DNA repair, cell cycle control, and aging will be studied. SIRT6 interacts with the 9-1-1 complex and has a role in BER by stimulating MYH, but inhibiting NEIL1 activity. We will test whether expression of DNA glycosylase can influence the sensitivity of Sirt6 deficient cells to DNA damage agents and whether SIRT6 can deacetylate hNEIL1. Because SIRT6 is required to regulate genomic integrity and impacts the aging process, revealing the mechanism of SIRT6 interaction in BER and cell cycle checkpoints is important. Successful completion of these studies will reveal important new information regarding the interactions among DNA repair proteins, cell cycle checkpoints, and an aging regulating protein. These studies will advance our understanding of carcinogenesis process and form the background work for the development of new anti-cancer drugs.